1. Field of the Invention
This invention relates to an optical recording medium for allowing an information to be accessed optically, and more particularly to an optical recording medium that is adaptive for recording information at a high density. Also, the present invention is directed to a recording and reproducing apparatus that is suitable for recording and reproducing a high density optical recording medium.
2. Description of the Related Art
The quantity of moving picture information, such as movie, has been reduced sharply in accordance with developments of digital picture processing techniques and moving picture compression techniques. For instance, analog video signals for two hours structured by a national television system committee (NTSC) system or a Phase alternating by line (PAL) system have an information amount of approximately 80 Gbytes, whereas digital video signals for two hours compressed by “MPEG-2”, which is a moving picture compression standard suggested by the moving picture expert group, have an information amount of approximately 15 Gbytes. As video signals become compressed to have such a small information amount, an optical recording medium, such as compact disc(CD) or digital versatile disc(DVD), also has been required to store about two hours worth of digital video signal information.
Meanwhile, an example of an optical recording medium having the largest recording capacity includes a DVD. Beam spots irradiated onto signal tracks of the DVD must have such a size that can minimize cross-talk caused by adjacent signal tracks. To this end, an optical system for recording and reproducing the DVD includes a semiconductor laser for generating a red laser beam with a wavelength of 650 nm, and an objective lens with a numerical aperture of 0.6. A DVD recorded using such an optical system is not suitable for recording two hours of moving picture information because only up to 4.7 Gbytes can be recorded.
In order to enlarge a recording capacity, there has been discussed a scheme of using a blue laser beam having a shorter wavelength than a red laser beam. A blue laser for generating such a blue laser beam will be commercially available soon in accordance with a development of GaN group laser. It has been known that this blue laser generates a laser beam with a wavelength of about 400 nm. An optical pickup including the blue laser can access an optical recording medium for blue laser, hereinafter referred to as “HD(high density)-DVD”, as well as an existent DVD. To this end, assuming that the HD-DVD have a substrate (i.e., light transmission layer) thickness of 0.6 mm (equal to that of the existent DVD) and that an optical pickup having a blue laser (hereinafter referred to as “blue laser pickup”) uses an objective lens with the numerical aperture of 0.6, because a diameter of beam spots irradiated onto a disc by the blue laser pickup has a dimension proportional to (λ/NA)2, the HD-DVD has 2.51 times the recording density of the conventional DVD. In other words, when the HD-DVD is manufactured in a form identical to the existent DVD, but has a recording capacity of 4.7 GB×2.51=11.8 GB.
Further, the blue laser pickup has an average optical aberration amount smaller than Marchel's criterion of 0.07λ so as to provide beam spots within a diffraction limit and an excellent signal-to-noise ratio(S/N). The average optical aberration amount is significant when it is given in a unit of wavelength, and which includes a spherical aberration, a comma aberration, a astigmatism and so forth. It has been known that such an average optical aberration amount is influenced by a comma aberration amount, which is inversely proportional to a wavelength(λ) of the beam. The comma aberration amount appears because of disc tilt, and is proportional to “t·(NA)3/λ”. In other words, the average optical aberration amount increases in accordance with a tilt amount of the disc. Accordingly, when the blue laser pickup has a tilt margin of ±0.6, which is equal to the existent optical pickup for DVD, the HD-DVD can not have the above-mentioned recording capacity. This is caused by the increased comma aberration caused by the decreased wavelength of the blue laser beam. For example, when a wavelength(λ) of the blue laser beam is 410 nm; the numerical aperture(NA) of an objective lens is 0.6; and a thickness(t) of a disc substrate is 0.6 mm, a recording capacity of the HD-DVD is reduced to approximately 8 to 9 GB.
As described above, it is difficult for an optical recording medium to have a recording capacity of 15 GB when using the blue laser. In order to solve this problem, there has been attempted a scheme of reducing a track pitch or a pit length. Various new control techniques have been applied so as to realize such a scheme. An example of the new control technique is described in a paper entitled “The path from DVD(red) to DVD(blue)”(JOINT MORIS/ISOM '97 Conference Proceeding pp. 52 to 53). In the paper, there is disclosed a scheme of carrying out a dynamic servo in accordance with a radial tilt angle to correct aberration and to raise recording density. However, if a tilt angle of a disc is generated in a radial direction, the same extent of tilt angle is generated in a tangential direction. Particularly, in the case of a disc having a serious surface vibration, the radial and tangential tilt angles become larger. Accordingly, an aberration correction through a tilt in the tangential direction is not made by carrying out only a dynamic servo in a radial direction.
An alternative technique for enlarging the recording density of an optical recording medium is described in a paper entitled “A rewritable optical disk system over 10 GB of capacity”(Optical Data Storage '98 Conference Edition pp. 131-133). This paper suggests a scheme of enlarging a recording density by raising the numerical aperture(NA) of an objective lens. As described in this paper, as the numerical aperture(NA) of an objective lens increases, a tilt margin of disc becomes enlarged. Also, in order to assure the tilt margin of the disc, a thickness(t) of the disc substrate must be reduced. For example, when the numerical aperture(NA) is set to 0.85, the disc substrate has a thickness of 0.13 mm in which a comma aberration amount(t(NA)3/λ) becomes approximately 1 so as to secure the tilt margin of the disc. According to the above-mentioned paper in which a thickness(t) of the disc substrate is set to 0.1 mm and the numerical aperture(NA) of an objective lens is set to 0.85, a HD-DVD capable of assuring 5.04 times the recording capacity of the existent DVD is provided. Moreover, the paper considers a reduced value caused by a comma aberration margin, a defocusing aberration margin and a spherical aberration margin, including a margin of substrate thickness, etc., and the paper states a HD-DVD having a recording capacity of about 20 Gbytes can be provided. However, the scheme disclosed in the paper has a problem in that the substrate thickness becomes very thin, about 0.1 mm. Further, a disc surface is weakened by dust and scratches, etc. due to this thin substrate. Also, an objective lens having the numerical aperture of 0.85 must not only combine two lenses, due to a difficulty in its manufacture, but also requires too short a working distance from the lens to the disc surface. It is further noted that a HD-DVD having a substrate thickness of 0.1 mm is required to configure a blue laser pickup capable of changeably accessing the existent DVD of 0.6 mm as well as a CD of 1.2 mm.